1. Field of the Invention
This invention relates to the NOx-adsorber aftertreatment system, and more particularly relates to utilizing engine-generated heat in NOx-adsorber aftertreatment systems.
2. Description of the Related Art
Emissions regulations on nitrogen oxides (NO and NO2, commonly termed NOx) have reached the point in many areas of the world where engine-out concentrations of NOx cannot be lowered to meet emissions standards. In some cases, aftertreatment devices are added to the exhaust system of the engine to further reduce NOx. One aftertreatment system to reduce NOx is the NOx-adsorber catalyst system (NAC), which intermittently adsorbs NOx (specifically the NO2 component of NOx), then desorbs (releases) the NOx while reducing it to other nitrogen compounds to meet emissions standards.
One challenge in NAC systems under the currently available technology is that the amount of NOx that can be stored on the NAC depends upon temperature. Further, NAC systems require periodic regeneration events to release and reduce the adsorbed NOx. These regeneration events require significant temperature to release the NOx, often more temperature than is naturally produced by the engine. A common method to produce the required temperature is to burn a hydrocarbon in the exhaust system, either by injecting extra hydrocarbon in the engine that does not combust within the cylinder, or by directly injecting hydrocarbon into the exhaust pipe with an auxiliary injector. However, in many applications, especially those where the engine does not spend a significant portion of the engine duty cycle under a significant work load (for example, in light duty applications), the engine does not produce enough heat to support burning hydrocarbons in the exhaust. This problem can be exacerbated by applications where the NAC must be placed at a significant distance from the engine due to the packaging requirements of the application the engine is installed within.
The current technology addresses the problem that normally engines do not heat up the NAC very quickly at startup. Therefore, a startup converter, or a lightoff converter, is installed in some systems. The startup converter usually comprises a small catalyst configured to respond quickly to engine temperature changes. This reduces the time after engine startup until the system reaches emissions-compliance, and it can reduce the time to enter a regeneration event once the event is commanded. However, the use of a startup converter does not make the system capable of achieving a regeneration where the engine exhaust temperature and packaging limitations do not introduce enough temperature to the NAC to initiate a regeneration.
Another problem under the current technology is a set of limitations imposed by the composition of the exhaust gas. While NAC systems can adsorb only the NO2 component of NOx, diesel engines—a primary producer of NOx emissions—produce mostly NO rather than NO2 when producing NOx. The catalyst on the NAC can convert some NO to NO2, and a pre-catalyst on the frontside of the NAC can also perform this conversion, but this reaction suffers from similar temperature issues as those experienced for the combustion of hydrocarbons.
A further problem imposed by the composition of the exhaust gas is the general lack of good reducing agents in the exhaust gas under typical engine operation. When the NAC is regenerated, hydrocarbons can be used as a reducing agent, and therefore the engine is typically operated in a “rich” configuration—or with less oxygen than stoichiometrically required to burn all of the fuel. However, hydrocarbons are an inefficient reducing agent for NO2. Carbon monoxide (CO) can be a more effective reducing agent, but it is difficult to produce in large quantities in a diesel engine, and there are emissions limits on the CO that can be emitted from the engine. Hydrogen (H2) is a very effective reducing agent for NO2, but the current technology available to generate H2 requires additional reforming equipment and/or very high temperatures (700 deg C. or greater) to achieve H2 production under currently available technologies.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that provides for improved NAC regeneration performance, particularly for engines that spend a large portion of the engine operating time at operating conditions where conventional NAC systems do not receive the temperature and exhaust gas composition to efficiently regenerate. Beneficially, such an apparatus, system, and method would utilize the engine-generated heat to assist the NAC in regenerating, and configure the exhaust gas composition to assist the NAC in achieving regeneration without the addition of expensive systems to reform hydrogen.